METABOLISM AND ENERGETICS

METABOLISM AND ENERGETICS
BIO 169 METABOLISM AND ENERGETICS CHAPTER 25 created by Dr. C. Morgan

TOPICS Introduction and Overview Carbohydrate Metabolism Lipid Metabolism Protein Metabolism Nucleic Acid Metabolism Metabolic Interactions Diet and Nutrition Bioenergetics

Introduction and Overview Objectives
Discuss the relationship between nutrients, the digestive system, the cardiovascular system, and cellular metabolism. Review the storage of excess nutrients. Review the definition of metabolism. Discuss the metabolic functions within cells. Discuss the roles of catabolism and anabolism.

Introduction Metabolism is the sum of all chemical reactions taking place in the body. Chemical reactions occur within the cytosol and organelles of cells and in the extracellular fluids. Our interest is in the ability of a cell to extract from nutrients sufficient chemical bond energy to produce ATP, the energy currency for cellular work. Additional high energy compounds are produced but will not be discussed in any detail. Oxygen, required for efficient nutrient use, is supplied to the blood by the lungs. The digestive system makes the nutrients available to cells via the cardiovascular delivery system.

Introduction (cont) Ideally, nutrient intake matches the metabolic demand of cells. If you take in more food than you need, it is stored in adipose tissue for subsequent use. Hormones, with some help from the nervous system, guide cellular metabolic activities, the storage of reserves, and the release of reserves from storage when they are needed. Nutrition is the absorption of nutrients from food. The fate of the absorbed nutrients will be the focus of our discussions.

Introduction (cont) Metabolic activities occurring within cells is cellular metabolism which performs several functions: * recycles cellular components (metabolic turnover) * provides energy for cell division and growth *carries out specialized activities of differentiated cells including secretion, contraction, nervous system communication via action potentials Catabolism is the breakdown of nutrients to harness energy in the chemical bonds of new molecules, especially high energy compounds such as ATP. For efficiency, catabolism occurs in many small steps which allows more released energy to be captured.

Introduction (cont) ATP produced from catabolic reactions links catabolism to anabolism, the building of new organic molecules needed by the cell to carry out its activities. New organic molecules are used for * ongoing maintenance and repair * growth * production of cellular secretions * building and storage of energy reserves. Catabolism and anabolism depend on the availability of a nutrient pool provided from digestion, absorption, and distribution.

Introduction (cont) Fig. 1

Relationships--Metabolic turnover and ATP production
Introduction (cont) Relationships--Metabolic turnover and ATP production Fig. 2

Overview of nutrient use in cellular metabolism
Introduction (cont) Overview of nutrient use in cellular metabolism Fig. 3

Carbohydrate Metabolism Objectives
Show the formula for glucose catabolism. Discuss glycolysis. Describe the production of ATP in mitochondria. Show the energy yield of glycolysis and aerobic cellular respiration. Describe alternative catabolic pathways when there is insufficient glucose available.

Carbohydrate Metabolism
From the nutrient pool cells first use the available carbohydrates, then the lipids, and amino acids third. Carbohydrate metabolism is mainly the story of glucose catabolism – C6H12O6 + 6O2  6 CO H2O There are two steps to the catabolism of glucose, glycolysis which occurs in the cytosol and aerobic respiration which occurs in the mitochondria. The useful products of glycolysis are 2 pyruvic acid molecules, 2 NADH, and 2 ATP for cellular use. The pyruvic acids and NADHs move into the mitochondrion for further processing which yields much more ATP.

Carbohydrate Metabolism (cont)
CYTOSOL Fig. 4

Needed for glycolysis are *glucose molecules *specific catalytic enzymes for each reaction *ATP, ADP, and inorganic phosphate (Pi) *NAD+ coenzyme carrier molecules that remove hydrogen atoms during one of the reaction steps The use of 2 ATPs prepares glucose for catabolism. NAD+ becomes reduced and takes on a hydrogen atom and one additional electron to become NADH. NADH moves into the mitochondrion where it will donate its electrons to the electron transport chain and H+ to solution.

Reduction of NAD+ H+ proton H+  oxidation of organic molecule More than one hydrogen atom may be removed at a time. NAD+ takes on 2 electrons, one proton, and leaves the other proton in solution  NADH + H+ FADH takes on one complete hydrogen (in mitochondria)

Phosphorylation of ADP occurs by two methods. *Substrate-level phosphorylation is the transfer of a high energy phosphate from a substrate molecule directly to ADP. ATP is made by substrate-level phosphorylation during glycolysis in the cytosol and in the TCA cycle which takes place in the mitochondrial matrix *Oxidative phosphorylation makes ATP using the electron transport chain, oxygen, and chemiosmosis (a special process occurring in the mitochondrion>90% of the ATP). Review: oxidation is the loss of electrons from a molecule; reduction is the gain of electrons by a molecule. During coupled oxidation–reduction reactions, hydrogens are also transferred to the carrier molecules.

Recall that it is electrons that participate in chemical reactions. Two types of carrier molecules, NADH and FADH2, that deliver hydrogen atoms and their electrons to the electron transport chain of the mitochondrion are important because * for each NADH delivered, 3 ATPs are generated and * for each FADH2 delivered, 2 ATPs are generated. The CO2 that is given off as waste originates from the carbons and oxygen in glucose. The only thing actually harvested from glucose is the energy from its chemical bonds and some hydrogens that are temporarily used to create an electrochemical gradient in the mitochondrion.

The mitochondrion is a double membrane organelle with an inner compartment containing a fluid called the matrix and an outer intermembrane space which also contains fluid. The outer membrane has large pores that allow ions, small molecules, and carriers to cross. A carrier protein contained in the inner membrane moves pyruvic acid molecules into the matrix. Each pyruvic acid is converted to an acetyl- Co-A (2 carbons) and in the process produces an NADH and a CO2 as waste.

TCA cycle completes the catabolism of glucose. (via GTP) 2 turns / glucose 20 Fig. 5

NADH and FADH2 donate hydrogens and electrons. The mitochondrial electron transport system (ETS) is a series of complex molecules that undergo coupled oxidation-reduction reactions. final electron acceptor Energy is used to move H+s into the intermembrane space. Fig. 6 a

Aerobic respiration generates most of the ATP Fig. 6 b

The donated electrons pass from one molecule to another with the hydrogen protons pumped to the outer compartment of the mitochondrion, creating an electrochemical gradient. As H+s pass back to the inner compartment through a special membrane protein, ATPs are made. Every 2 H+s passing back into the matrix provides enough energy to produce one ATP by the special protein. Oxygen is the last electron acceptor where it also combines with the H+ to form metabolic water. This is where you consume oxygen. With insufficient oxygen, electron transport stops. Total = 36 ATP from each glucose using glycolysis + ETS.

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Glycogen, triglycerides, and proteins are storage forms of energy. When there is insufficient glucose, triglyceride fragments and amino acids are able to enter glycolysis or the TCA cycle at various points to be catabolized. Skeletal muscle and the liver store glycogen.

Overview Glucose may be synthesized from other starting materials in a process called gluconeogenesis.

Glycogenesis stores excess glucose as glycogen in the liver and skeletal muscle. Glycogenolysis breaks down glycogen stores to yield glucose for metabolism. Note interconversions to intermediates of glycolysis. 27 Fig. 8

Lipid Metabolism Objectives
Discuss the mechanisms for lipid catabolism. Describe the importance of lipids as energy reserves. Discuss the synthesis of lipids. Describe the transport and distribution of lipids. Discuss the relationships of dietary fats and cholesterol in health.

Lipid Metabolism Lipolysis is catabolism of lipids and beta-oxidation is the catabolism of fatty acids. More energy is gained from the catabolism of a gram of lipids than either carbohydrates or proteins. Triglycerides represent most of the lipids stored in the body. A triglyceride is catabolized into its component parts— a glycerol molecule and three fatty acid chains. The glycerol molecule enters the TCA cycle after it is converted to pyruvic acid by enzymes in the cytosol.

Lipid Metabolism (cont)
Beta-oxidation is the catabolism of fatty acid chains. 2 C fragments are removed and converted to acetyl- CoA molecules which then enter the TCA cycle. ATP must be used in the conversion but reduced carriers are formed. Beta oxidation 31 Fig

Lipids are the “best” energy stores because they provide large amounts of ATP, are stored in compact droplets contained in the cytosol, and are not degraded by water soluble enzymes. Oxygen is required since lipid catabolism occurs within the mitochondrion by aerobic metabolism. Because their catabolism requires more time, lipid reserves are catabolized during times of rest (muscles) to save valuable glycogen and glucose. Skeletal muscles cycle between carbohydrate and lipid metabolism depending on immediate demand.

Lipid synthesis (lipogenesis) involves the synthesis of glycerol from intermediates of glycolysis and the synthesis of fatty acid chains from acetyl-Co A fragments. Not all fatty acids are able to be synthesized by cells. Linoleic acid and linolenic acid molecules cannot be synthesized so must be obtained from the diet. Such a dietary requirement is an essential fatty acid. These fatty acids which come from plant sources are incorporated into cell membranes and are used to make prostaglandins.

Lipid Metabolism (cont) Lipid Synthesis – lipogenesis
Lipids may be synthesized from most nutrient fragments. Fig. 10 34

All cells need lipids for their cell and organelle phospholipid membranes and lipids are needed by some cells to build steroid hormones. Free fatty acids are able to diffuse into cells. Because they are insoluble, lipids circulate as lipoproteins which consist of large glycerides + cholesterol coated with phospholipids and proteins. The lipoprotein complexes, mostly made in the liver, are water soluble and transported in the plasma. The proteins on the outer surface are recognized by cell receptors which determines cellular absorption of particular lipids contained within lipoprotein complexes.

Lipoproteins are grouped according to their size and relative amount of lipid compared to protein. Chylomicrons are the largest (0.03 – 0.05 m) with 95% of their weight triglycerides. Recall these are made by villi cells of the small intestine. Very low-density lipoproteins (VLDLs) (0.025 – m) contain a large proportion of triglycerides and some phospholipids + cholesterol that are delivered to peripheral tissues. Intermediate-density lipoproteins (IDLs) contain less triglycerides and more phospholipids + cholesterol in proportion to protein.

High-density lipoproteins (HDLs) (10 nm) contain about equal proportions of lipid and protein with the lipids mostly cholesterol and phospholipids (not triglycerides). HDLs are moving materials from peripheral tissues back to the liver for storage or excretion in the bile. These lipids do not accumulate in vessels so sometimes they are called the “good cholesterol”.

Lipid Metabolism (cont) Lipid transport and utilization
Fig. 11 a Capillaries that serve skeletal and cardiac muscle cells, hepatocytes, and adipose cells have a lipoprotein lipase that releases fatty acids and glycerides from chylomicrons. Fatty acids and glycerides move into the interstitium. 38

Lipid Metabolism (cont) Lipid transport and utilization
move into cells by endocytosis Liver controls all other lipoprotein distribution Fig. 11 b

Dietary fats and cholesterol are the subjects of discussion. Atherosclerosis (plaque buildup in arteries) is directly related to elevated cholesterol levels in the blood. Dietary cholesterol should be less than 300mg / day. Only 20% of circulating cholesterol comes from dietary sources and the rest comes from interconversions from dietary saturated fats since excess lipids are made into acetyl-Co As which are then made into cholesterol. Thus, reducing overall fat intake, especially saturated fats, helps reduce circulating cholesterol levels. Genetic factors, age, and physical conditioning all affect cholesterol levels.

If there is no family history of CAD, a blood cholesterol level below 200 mg / dl is safe (no lifestyle changes needed). A level >240 mg / dl requires major lifestyle changes. A level > 350 mg / dl requires drug therapy. Blood HDL levels may also be measured and the LDL level calculated. High total cholesterol plus elevated LDL (or low HDL) is linked to the development of atherosclerosis. The number of known risk factors for CAD along with blood lipid levels determines the recommendations for lifestyle changes or drug therapy.

Protein Metabolism Objectives
Recall the variety of proteins in the body. Describe the events in amino acid catabolism. Discuss why proteins are not quick energy sources. Discuss protein synthesis.

Protein Metabolism Your body contains more than 100,000 different proteins assembled from 20 different amino acid monomers. There is a constant turnover of cellular proteins that occurs by enzymes present in the cytosol. Mitochondria are able to make use of some amino acids in the TCA cycle if carbohydrates and lipids are lacking. Amino acids enter the TCA cycle at different points so the number of reduced carriers generated varies. The average ATP yield per gram of protein is comparable to that for carbohydrates.

Protein Metabolism (cont)
The first step of amino acid catabolism is the removal of the amino group (–NH2) which requires vitamin B6. If the amino group is transferred to a keto acid molecule (a double bonded oxygen on the middle carbon) to create a new amino acid, the process is called transamination.

The amino group may be removed by deamination which also generates an ammonium ion or a toxic ammonia molecule. or NH3

Most deamination occurs in the liver which is able to convert the ammonia into nontoxic urea that is excreted in the urine. Fig. 12 c

The body is able to synthesize 10 amino acids. There are 10 essential amino acids which must be obtained in the diet of which 2 are nonessential amino acids that may be synthesized but only in insufficient quantities. Even though we do not generally use proteins as energy sources, the diet must have a balance of proteins to supply the amino acids needed for the synthesis of structural and functional proteins. In some third world nations, there are a number of protein deficiency diseases linked to malnutrition. Kwashiorkor and marasmus are two protein deficiency diseases.

The amino acids needed for protein synthesis are from dietary sources, transamination reactions, and amination reactions— the addition of an amino group to a short chain carbon molecule. Fig. 13

Some genetic disorders arise because DNA does not code for enzymes needed for amino acid metabolism. Phenylketonuria (PKU) is a disorder where the enzyme to convert phenylalanine to tyrosine is defective. Tyrosine is needed to synthesize NE, E, dopamine, and melanin. Nervous system developmental problems will occur in infants and young children if PKU goes undiagnosed. When too much phenylalanine builds up in the blood, it interferes with synthesis of other proteins.

Nucleic Acid Metabolism Objectives
Recall the forms of cellular nucleic acid molecules. Discuss RNA catabolism and recycling. Describe the synthesis of nucleic acids. Summarize metabolic pathways.

Nucleic Acid Metabolism
DNA and RNAs represent the nucleic acids present in the cell (polynucleotide = base+5Csugar+P). The DNA is confined to the nucleus and is perpetual. Three types of RNA, mRNA, tRNA, and rRNA are involved in protein synthesis which occurs in the cytosol. RNAs are continually recycled. First, RNA polynucleotides are broken down into individual nucleotides by appropriate enzymes. Nucleotides may be further catabolized into sugars and nitrogenous bases but more commonly they are simply recycled into new macromolecules.

The only useful molecules from the catabolism of RNA are the sugars which enter glycolysis and the nitrogenous bases uracil and cytosine which may be converted to acetyl-CoA. The nitrogenous bases guanine and adenine cannot be catabolized so are deaminated to form uric acid which is excreted in the urine. Normal plasma uric acid concentrations are 2.7 – 7.4 mg/dl. Increases > 7.4 mg/dl cause body fluids to be saturated with uric acid crystals which results in gout. The buildup is usually linked to kidney function problems.

DNA synthesis occurs only prior to cell division. RNA synthesis occurs continuously . mRNA synthesis is controlled in part by hormones which may permit, block, or change the amount of mRNA synthesis. mRNA molecules are degraded several minutes to a few hours after they appear in the cytosol. tRNA and rRNA molecules persist much longer—typically 5 days. Specific enzymes catalyze all the reactions that synthesize nucleic acid molecules.

Cellular Metabolism Summary
Fig. 14

Metabolic Interactions Objectives
Discuss the body locations where distinct metabolic components are present based on the enzyme composition of the cell. Describe the absorptive state of the body. Describe the postabsorptive state of the body. Define obesity. Discuss ketoacidosis and its link to diabetes.

Metabolic Interactions
The liver, adipose tissue, skeletal muscle, neural tissue, and the remaining tissues of the body have distinctive metabolic demands and interactions. The liver, extremely important in metabolic regulation and control, contains a huge diversity of metabolic enzymes. Hepatocytes are able to catabolize and synthesize all classes of macromolecules and can store glycogen. The liver adjusts the level of circulating nutrients. Adipose tissue, scattered throughout most of the body, stores triglycerides which may be mobilized. Skeletal muscle stores glycogen for its own use and muscle proteins may be catabolized in extenuating circumstances.

Metabolic Interactions
Neural tissue must be supplied with glucose continuously in order to maintain its function since it does not store energy reserves. Other peripheral tissues are able to metabolize a variety of nutrients but do not store energy reserves. During a daily routine, there are two patterns of metabolic activities: the absorptive state and the postabsorptive state. The absorptive state follows the digestion and absorption of nutrients from the intake of food. If you eat three meals a day, you spend about 12 hours in the absorptive state with insulin the main hormone since it stimulates glucose movement into cells.

Metabolic Interactions (cont)
Absorptive State Hormones regulate metabolism (TABLE 1) Fig. 15

During the absorptive state, liver hepatocytes adjust circulating glucose and amino acid levels in blood arriving via the hepatic portal circulation. Insulin is the most important absorptive state hormone. Insulin stimulates liver cells to take up glucose so the blood level remains relatively stable (90 – 110 mg / dl). The liver uses what it needs and stores the rest as glycogen. Amino acid levels range between 35 and 65 mg / dl. The liver uses the absorbed amino acids for protein synthesis. Absorbed lipids move into the lymphatic circulation so are taken up by other tissues rather than the liver.

Absorptive state (cont) Adipose tissue removes fatty acids and glycerol for 4 to 6 hours after a meal high in fats. Adipocytes also remove glucose and amino acids as needed for synthetic activities. Adipocytes will increase in size to accommodate the the need for more storage space when intake > use. Insulin stimulates all other tissues to absorb and use glucose. Also when glucose levels are high, most tissues do not absorb circulating lipids—leaving all of them for adipocytes. Muscle cells store between 0.5 and 1% of their weight as glycogen.

During the postabsorptive state there is no nutrient absorption occurring along the digestive tract so cells adjust their activities accordingly. Metabolic reserves are mobilized to meet cellular needs. Most reserves are lipids stored as fat. glucagon, epinephrine, and glucocorticoid hormones Fig. 16

Postabsorptive State Fig. 17

Postabsorptive state (cont) At a blood glucose of 80 mg / dl, liver cells turn to glycogenolysis under the influence of glucagon and epinephrine hormones. The liver stores 75 – 100 g of glycogen which will maintain blood glucose levels for about 4 hours at rest. When the blood glucose level falls below 70 mg / dl, liver cells begin to engage in gluconeogenesis under the influence of glucocorticoids from the adrenal cortex. During the postabsorptive state, liver cells will absorb fatty acids and glycerol from the blood to provide energy. Some molecules of acetyl-CoA generated by beta oxidation are converted to ketone bodies used in peripheral tissues.

Postabsorptive state (cont) Deamination and transamination of amino acids also takes place in the liver with the generation of ketone bodies. There are three types of ketone bodies, acetoacetate, acetone, and betahydroxybutyrate. These are acids that enter the general circulation. Blood concentrations >30 mg / dl indicates ketonemia, an indication of ketosis from lipid and amino acid metabolism. If ketones appear in the urine it is called ketonuria. Acetone may be evident in the breath of an individual in uncontrolled diabetes mellitus if they are in a state of diabetic ketoacidosis.

Postabsorptive state (cont) Rising circulating levels of epinephrine, glucocorticoids, and growth hormone stimulate adipocytes to mobilize their fat reserves to the blood. You have a 2 month supply with 50% in the hypodermis. Skeletal muscle will use its stored glycogen reserves plus fatty acids and ketone bodies that have been absorbed. Other peripheral tissues will also eventually absorb fatty acids and ketone bodies when glucose is exhausted. Neural tissue relies on a continual glucose supply. Only in advanced starvation will neural tissue utilize ketone bodies and lactic acid (from muscle cells).

Diet and Nutrition Objectives
Describe the food groups and food pyramid. Discuss nitrogen balance in the body. Discuss the need for minerals. Discuss the need for and types of vitamins. Describe relationships between diet and disease. Discuss some age related diet and nutrition concerns.

Diet and Nutrition Nutrition is the absorption of nutrients from food. Normal cellular homeostasis depends on the delivery of adequate nutrients, O2, and removal of wastes. Your day-to-day nutritional requirements vary. A balanced diet will contain all nutrients needed to maintain homeostasis and sufficient water for fluid balance. An unbalanced diet will lead to malnutrition. There are four to six basic food groups (depending on your preferred groupings) from which daily food selections should be made.

Diet and Nutrition (cont)
Food Pyramid TABLE 2 Fig. 18 72

You need nitrogen for the synthesis of amino acids, nitrogenous bases of nucleic acids, creatine in muscle cells, and the porphyrin rings of hemoglobin, myoglobin, and cytochromes (ETS). To meet the need for nitrogen, you must have an adequate dietary intake because you do not store nitrogen reserves. You are in positive nitrogen balance if intake matches use plus excretion in the urine and feces. During starvation, nitrogen compounds are not catabolized from cells until all other compounds are used.

Minerals are elements other than carbon, hydrogen, oxygen, or nitrogen. They are inorganic ions released as compounds dissociate. Mineral ions are important for many reasons (TABLE 3): *Sodium and chloride are osmotically important in the maintenance of fluid homeostasis within compartments. *Ions maintain the resting membrane potential in excitable cells. *Ions conduct actions potentials in excitable cells. *Ions release neurotransmitters. *Ions act as cofactors for the many enzymatic reactions (recall the clotting mechanism).

Ions (cont) *Ions form part of the skeleton. *Ions transport respiratory gases (HCO3–). *Ions act as buffers. Most vitamins are used as cofactors in enzyme reactions. There are two vitamins groups, fat-soluble and water- soluble. Vitamins A, D, E, and K are fat-soluble which means they will be absorbed with other lipids (in micelles) along the digestive tract (see TABLE 4).

Inadequate intake of certain fat-soluble vitamins is responsible for some disorders (recall rickets from a vitamin D3 deficiency). Excessive intake of vitamins may lead to hypervitaminosis which may adversely affect liver functions. Water-soluble vitamins include the B vitamins, niacin, and vitamin C, all components of coenzymes (TABLE 5). Niacin and vitamin B2 form part of the important carrier compounds NAD and FAD that shuttle hydrogens and electrons to the transport chain in mitochondria. Several B vitamins are required for proper growth. Water-soluble vitamins are not stored in any quantity.

You need 2000 – 2500 ml of water intake per day of which about 48% comes from the solid foods. Dietary problems in the U.S. center on an imbalance in the diet. Americans tend to eat too much of the wrong foods. We eat too much fat which contributes to heart disease, obesity, atherosclerosis, hypertension, and diabetes. Two eating disorders involve inadequate nutrition. Anorexia nervosa (self-induced starvation) and bulimia (binging and induced vomiting) are most common among adolescent females.

Nutritional requirements do not change with age but caloric intake requirements do decrease by about 10% for each decade past age 50. There is an increased need for calcium in the diet for bone homeostasis (and supplementary vitamin D3 for people indoors most of the time). Malnutrition is not uncommon in the elderly who lose their desire to eat because food does not taste good any more and shopping and food preparation are difficult. Digestive activities also decline with age.

Bioenergetics Objectives
Define bioenergetics and units of energy. Discuss metabolic rate and its measurement. Describe the concept of thermoregulation. Present the mechanisms for heat transfer. Discuss regulation of heat gain and loss. Describe some individual variations with respect to thermoregulation.

Bioenergetics Energy is released as chemical bonds of nutrient molecules are broken during catabolism. The unit of measurement for this energy is the calorie. One calorie is the amount of heat energy needed to raise the temperature of 1 gram of water 1 degree centigrade. It is more practical to measure our food energy as packets of 1000 calories, the kilocalorie (kc or C) which is the Caloric value seen on food packaging labels. Each Calorie will raise 1 kilogram of water 1 degree centigrade. Lipids have 9.46 C / gram; carbohydrates = 4.18 C / g; proteins = 4.32 C / g.

Bioenergetics (cont) The Caloric value of a gram of food can be measured by placing a known quantity in a special chamber (calorimeter) where it is oxidized by burning. The calorimeter is surrounded by a known quantity of water. The temperature change in the water represents the Caloric value of the food item. Most foods are a mixture of carbohydrates, lipids, and proteins.

Bioenergetics (cont) Your metabolic rate is your energy utilization in Calories / selected unit (usually per hour). Basal metabolic rate (BMR) is your resting utilization rate. The average BMR is around 70 C / hr or 1680 C / day. Daily requirements are determined by activity level and individual metabolic differences. Appetite is poorly understood. Psychological and hormonal factors are important in appetite regulation and control. Leptin, a hormone released by adipocytes that are synthesizing triglycerides, binds to neurons in emotional centers that suppress appetite.

Bioenergetics (cont) Energy released during metabolism that is not harnessed in the chemical bonds of other compounds is released as heat. Since metabolism is only 40% efficient, it generates great amounts of heat as a byproduct. If body temperature is not maintained, serious problems will arise. Chemical reactions increase as temperature increases and vice-versa; enzymes denature at temperatures above 40oC. Thermoregulation is the homeostatic process that maintains body temperature within acceptable limits.

Bioenergetics (cont) 85

Bioenergetics (cont) You exchange heat with your surroundings by four mechanisms: radiation, conduction, convection, and evaporation. *radiation is the loss of heat into the space around you (50%) *conduction is heat transfer by contacting a surface that is at a different temperature than your body (10% of loss) *convection is movement of warm air away from your body surface and its replacement by cooler air creating a current (15% of loss) *evaporation of water molecules from your body surface and lungs removes heat (20% of loss) Ideally, heat loss = metabolic heat generated or heat gained.

Bioenergetics (cont) Thermoregulation is accomplished by the hypothalamic heat-loss and heat-gain centers. The heat-loss center output is via the parasympathetic n.s. and the heat-gain center output is via the sympathetic n.s. To promote heat-loss, output from the loss center causes *dilation of peripheral blood vessels (inhibition of the sympathetic output) which promotes heat loss by radiation and convection *stimulation of sweat glands to promote heat loss by evaporation (not effective in high humidity conditions) *an increase in rate and depth of ventilation

heat conservation by counter current exchange in large vessels
Bioenergetics (cont) To restrict heat loss, heat gain center causes peripheral vascular constriction heat conservation by counter current exchange in large vessels Fig. 20

Bioenergetics (cont) To promote heat production, the heat-gain center causes *shivering thermogenesis of skeletal muscles which may increase heat generation up to 400% *nonshivering thermogenesis which involves the secretion of hormones that increase metabolic activity of cells and nutrient conversions to support it. The hormones most involved are epinephrine and thyroid hormones via stimulation of hypothalamic TRHTSH.

Bioenergetics (cont) Hypothermia is induced during open heart surgery in order to decrease the metabolic demands of cardiac tissue. Up to three hours of inactivity may be tolerated by an adult heart when it is cooled to about 150C. Individuals may become acclimatized to temperature variations. Surface to volume ratios affect heat loss. Large individuals do not lose heat as efficiently as smaller individuals. Infants cannot shiver and their body temperature is less stable than in adults. Infants do have brown fat consisting of adipocytes in the upper body that are innervated by sympathetic neurons

Bioenergetics (cont) Sympathetic outflow increases metabolism in brown fat and will generate heat proportionately. Two people weighing the same will differ in thermal responses. If much of the weight is adipose tissue, they are well insulated and have more difficulty with heat loss than an individual with a large muscle mass. The settings of hypothalamic temperature centers may also vary throughout a 24 hour period among individuals. Elevated body temperature is pyrexia and a fever is a sustained body temperature above 37.20C.

The End TOPICS Introduction and Overview Carbohydrate Metabolism
Lipid Metabolism Protein Metabolism Nucleic Acid Metabolism Metabolic Interactions Diet and Nutrition Bioenergetics The End

METABOLISM AND ENERGETICS

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